Angewandte Chemie International Edition

sp-Hybridized carbon allotropes based on a polyyne framework are well documented, while properties of the cumulenic form of these allotropes remain mysterious. In their Communication on page 1817 ff., R. R. Tykwinski and co-workers shed new light on the properties of cumulenes (such as bond length alternation) through the synthesis, spectroscopic study, and crystallographic analysis of cumulenes that contain up to nine consecutive double bonds (i.e., a [9]cumulene). Cover art: Annie Tykwinski; based on a concept from D. Wendinger.

Understanding of gas adsorption mechanisms at the molecular level is vital for the rational development of compounds that can remove harmful greenhouse gases from the atmosphere. In their Communication on page 1692 ff., J. Li and J. B. Parise et al. describe the adsorption of CO₂ molecules in a “pocket” between two aromatic rings of a nanoporous coordination network. The CO₂–framework interaction is characterized by a combination of methods.

The synthesis of chrysaorole , which is a jellyfish-shaped heteroaromatic molecule, is described by D. Myśliwiec and M. Stępień in their Communication on page 1713 ff. This strained system was obtained from a carbazole-based macrocyclic precursor by using a new “fold-in” strategy.

Colored stable biradicals are described by H. W. Roesky, G. Frenking, I. Tkach, D. Stalke, and co-workers in their Communication on page 1801 ff.; they were prepared from SiCl₂, which is stabilized by an N-heterocyclic carbene, and a cyclic alkyl(amino)carbene. The deep-blue crystals of one polymorph of the product are stable upon exposure to air for about a week, while the solution in THF decomposes rapidly when exposed to air. A CC bond formation can be observed in a side reaction of the synthesis.

“Chemistry is fun because it creates its own object (Berthelot). My favorite way to spend a holiday is with a rental car, lots of time, and no itinerary …” This and more about Mathias Christmann can be found on page 1624.

Wiley-VCH, Weinheim, 2012. 688 pp., softcover, € 69.00.—ISBN 978-3527326334

All aglow: Both a fluorescent biosensor, composed of a circularly permuted yellow fluorescent protein (cpYFP), and a small-molecule fluorescent probe (1) for the detection of CO in living cells have been recently reported. Though different these novel probes were designed based on the unique binding ability of CO to transition-metal ions.

Topologically stable magnetic screw-like nanostructures called skyrmions were designed by using the concept of topology and the guidance of theory. These particles in real space are found in non-centrosymmetric compounds such as MnSi. Skyrmions might have an enormous impact in the context of future spintronics applications.

Silver shield: Silver is used in different forms as an antibacterial agent. Earlier, sparingly soluble silver salts were predominantly used, but today, silver nanoparticles (see picture for an SEM image of cubic silver nanoparticles) are gaining increasing importance. As silver is also toxic towards mammalian cells, there is the question of the therapeutic window in the cases of consumer products and medical devices.

Stretching the role of a conductor: A new kind of polyurethane sponge–Ag nanowire–poly(dimethylsiloxane) (PUS-AgNW-PDMS) stretchable conductors can be easily fabricated based on a novel binary network structure design, which greatly enhances their electronic performance. These PUS-AgNW-PDMS elastomeric conductors show improved electromechanical stability compared to previously reported stretchable conductors.

A Missing Gold Link: An Au^III alkene complex has been prepared and characterized by NMR spectroscopy and X-ray crystallography. Its bonding features have been analyzed by DFT calculations and natural bond orbital (NBO) analysis. In [(cod)AuMe₂]⁺, the unequal AuC bond lengths result from the domination of the preference of 1,5-cyclooctadiene (cod) for nonparallel double bonds over back donation from the metal which favors parallel double bonds.

Electrochemical energy storage: The performance of MnO₂ as a pseudo-capacitive material was enhanced by doping electrodeposited MnO₂ with physically deposited gold atoms (see picture). The resulting MnO₂ electrodes showed an enhanced electronic conductivity and a remarkable stability under voltammetric cycling.

One of four: A chiral phosphoric acid enables asymmetric hydrogenation of imines with an achiral iridium catalyst by virtue of noncovalent interactions. These interactions lead to the formation of a highly organized ternary complex, and the hydride is transferred highly enantioselectively.

The huge pool of hypothetical zeolites can be screened for likely candidates for successful targeted synthesis on the basis of a set of reliable criteria derived from the observation that the local interatomic distances in all existing zeolites strictly obey several rules. For example, the average TT and TO distances (<D_TT> and <D_TO>; T is Si, Al, P, or another element) in existing zeolites (graph region highlighted in cyan) have a linear relationship.

Magic rubber bullets: Novel anticancer nanoparticles made of well-defined polymer–drug conjugate amphiphiles are prepared by the controlled growth of a hydrophobic polyisoprene chain from a drug macroinitiator by using nitroxide-mediated polymerization (NMP; see picture). The resulting conjugates self-assembled into nanoparticles exhibiting high drug payloads and significant anticancer activities both in vitro and in vivo.

Nanoparticles on demand: Upon the site-specific covalent attachment of hydrophobic molecules to one end of the biopolymer backbone, chimeric polypeptides (derived from elastin-like polypeptides) can self-assemble to form thermoresponsive nanoparticles suitable for drug delivery. Molecules with a distribution coefficient greater than 1.5 imparted sufficient amphiphilicity to drive self-assembly into sub-100 nm nanoparticles (see picture).

Quick: An exogenously induced quick increase of the H₂S concentration (80 s) in MCF-7 cells can be visualized by ratiometric imaging using a new probe (CouMC) that can target mitochondria. CouMC was constructed by combining merocyanine and coumarin fluorophores. The selective nucleophilic addition of HS⁻ to the merocyanine derivative at neutral pH is crucial for the rapid H₂S detection.

Trapped in a porous material: The position of adsorbed CO₂ in a nanoporous coordination framework was determined using a combination of techniques including single-crystal X-ray diffraction, in situ X-ray powder diffraction coupled with differential scanning calorimetry, and theoretical calculations. The study reveals that the adsorbed CO₂ stays in a “pocket” between two phenyl rings, interacting with the aromatic electron density (see picture).

A NON-issue: Potassium complexes of an extremely rigid and sterically encumbered NON-donor ligand have been prepared, and the solid-state structures (see figure) feature remarkably short potassium–alkane distances. DFT calculations highlight the presence of an electrostatic cation-induced dipole potassium–alkane interaction supported by interactions between the alkane and the surrounding ligand framework.

Substrate specificity: Biochemical and crystallographic analyses reveal the hypoxia-inducible factor hydroxylase (FIH) as being promiscuous with respect to the residues that it can hydroxylate in β-position, which in addition to Asn, Asp, and His include Leu and Ser residues. The Ser substrate is oxidized to an epimeric β-geminal diol product (see picture).

It's a SnAP! The transformation of aldehydes into N-unsubstituted 3-thiomorpholines provides a convenient alternative to metal-catalyzed cross-coupling reactions, which are generally unsuited to the functionalization of saturated N-heterocycles. A copper-mediated radical cyclization is the key to the mild conditions, high functional group tolerance, and broad substrate scope offered by these reagents.

Just add water: A cube-like hydrogen-bonding network in a fluorescent supramolecular cluster dynamically deforms upon the specific addition of a water molecule. The deformation is amplified through the conformational change of the cluster to result in the transformation of host frameworks. This transformation provides water-responsive guest-exclusion and fluorescent-modulation behaviors.

Molecular jellyfish: A new family of bowl-shaped aromatic compounds swims into view (see picture). Quite unlike true jellyfish, chrysaoroles possess a rigid skeleton, which is assembled from fused carbazole units. Their synthesis involves a fold-in step to convert a macrocyclic precursor into the bowl-shaped target molecule.

Playing by the rules: Combinatorial domain swaps among “deconstructed” non-reducing polyketide synthases (NR-PKSs) revealed the rules behind product assembly (see scheme). The control exerted by individual catalytic domains was found to be sufficiently great that heterocombinations of domains from different NR-PKSs synthesized products in a predictable manner.

Twice as nice: Total syntheses of dimeric alkaloids, (−)-complanadines A (1) and B (2), were achieved from (−)-lycodine. The unsymmetrical motif was assembled through direct arylation of the pyridine N-oxide. The absolute configuration and specific rotations of complanadines A and B were identified. Cbz=Benzyloxycarbonyl.

Two approaches to the total synthesis of the dimeric Lycopodium alkaloid complanadine B have been achieved. In the first approach (see scheme; route 1), a keto lycodine unit is coupled to another lycodine unit whereas in the latter approach (route 2), selective oxygenation of one of two pseudo-benzylic positions is achieved.

Enantioselective: The first highly enantioselective kinetic resolution of 3-hydroxy-3-substituted oxindoles has been developed through oxidative esterification catalyzed by a N-heterocyclic carbene (see picture). This method uses a simple procedure and provides 3-hydroxy-oxindoles with various substituents at the 3-position in excellent enantiopurity. S=selectivity.

Going flat out: Simultaneous modifications of the textural, surface, and electronic structures of a rigid conjugated carbon nitride polymer has been achieved using direct co-condensation of urea and Ph₄BNa. This method gives boron-doped carbon nitride nanosheets (see picture) that optimize the capture of light, improve the charge-separation kinetics, and enhance the surface reactivity for hydrogen photosynthesis.

Smooth salen: Ru(CO)–salen complex 1 is an effective catalyst for asymmetric benzylic and allylic CH bond amination using 2-(trimethylsilyl)ethanesulfonyl azide (SESN₃) as the nitrene source. The reaction proceeded with high enantioselectivity and excellent regioselectivity. An ethyl group can be selectively aminated, even in the presence of an n-propyl group. No migration or isomerization of the double bond was observed.

Michael brings the rings: An asymmetric intramolecular oxa-Michael reaction involving iminium activation has been developed. This reaction provides enantioenriched 1,4-dioxane derivatives with up to 99 % yield and 98 % ee. The method allows for concise and stereoselective access to stereodiverse, complex tetracyclic compounds containing a bicyclo[2.2.2]octan-2-one backbone with multiple chiral centers.

A ‘F'lurry of activity: The title reaction provides a convenient procedure for direct synthesis of perfluoroalkylated phosphines. The synthesized phosphine n-C₁₀F₂₁PPh₂ forms a complex with palladium(II) to give 1 and several runs of coupling reactions are attained with n-C₁₀F₂₁PPh₂ by using a fluorous/organic biphasic system.

A new twist: A one-pot nitrous acid free, diazonium-free, and transition-metal-free variation of the Fischer indole synthesis has been developed. Condensation of quinone monoketals and aliphatic hydrazine hydrochlorides afforded indoles via intermediate alkylaryldiazenes. This method will complement the classical Fischer indole synthesis by providing indoles in two steps from widely available phenols under mild conditions.

It's complex: Ruthenium-catalyzed [3+2] cycloaddition of ethynylcyclopropanes with aldehydes and aldimines has been found to give the corresponding 2-ethynyltetrahydrofurans or -pyrrolidines in high to excellent yields. In both cases, the formation of a ruthenium allenylidene complex as a key reactive intermediate is supported by density functional theory calculations. Cp*=η⁵-C₅Me₅.

Count against: Stable and planar antiaromatic, expanded vinylogous isophlorins encourage chalcogen–fluorine noncovalent interactions. The wide cavity of the macrocycle accommodates covalently bound fluorine substituents, thus leading to multiple nonbonding interactions

Rings beget rings: Benzazepines, well-known structural design elements in medicinal chemistry, are readily prepared by a one-pot palladium-catalyzed oxidative cycloaddition of isatins with various alkynes.

Hydrogen abstraction in the spotlight: UV irradiation (365 nm) of dinuclear ruthenium tetrahydride, [(Cp*Ru)(μ-H)₄(RuCp*)] (Cp*=η⁵-C₅Me₅), in a 2-alkanone, such as acetone or 2-butanone, resulted in the formation of dinuclear oxatrimethylenemethane complexes, [(Cp*Ru)(μ-η³:η¹-CH₂COCHR)(μ-H)₂(RuCp*)] (R=H and CH₃), through hydrogen abstraction from the α and α′ positions of the carbonyl group.

Water is out, aryl is in! Asymmetric synthesis of isoindoline-1-ones bearing an α-triaryl-substituted stereogenic center was realized in the enantioselective addition of arylboroxines to 3-aryl-3-hydroxyisoindolin-1-ones in the presence of a hydroxorhodium/chiral diene catalyst, where cyclic N-carbonyl ketimines were generated in situ by dehydration.

Gold brings us together: Taking advantage of the orthogonal reactivities of Au^I and Au^III towards CH activation of electron-poor and electron-rich arenes, respectively, a novel approach for the synthesis of biaryls through double CH activation is proposed. Stoichiometric studies demonstrate that these oxidative couplings occur with high selectivity at low temperature.

How to train your silane: A new family of chiral copper(I) complexes that bear a bifluoride counteranion were prepared and used in the first example of the enantioselective transfer of a silyl group to an aldehyde. This procedure provides fast access to non-racemic α-hydroxysilanes in high enantioselectivities.

Ironing out a solution: Chemical and visible-light-driven water oxidation by iron complexes and iron salts at pH 7–9 has been investigated. The iron complexes and salts act as precatalysts (see scheme) to produce α-Fe₂O₃ nanoparticles that are the real catalyst for the water oxidation with a turnover number over 1000.

Poring over polymers: Highly porous polymers with hierarchical macro- and mesoporous structures were synthesized by using the title method and UV radiation. The porosity properties of the polymers can be easily tuned by controlling the CO₂ pressure used, and the polymers have potential applications in catalysis. IL=ionic liquid, PAM=polyacrylamide.

Pick and choose: Novel cationic diaza-, azaoxo-, and dioxo[6]helicenes are readily prepared and functionalized selectively by orthogonal aromatic electrophilic and vicarious nucleophilic substitutions (see scheme). Reductions, cross-coupling, or condensation reactions introduce additional diversity and allow tuning of the absorption properties up to the near-infrared region. The diaza salts can be resolved into single enantiomers.

Silicon becomes colored: Stable biradicals were prepared from an N-heterocyclic carbene stabilized SiCl₂ and a cyclic alkyl(amino)carbene, and characterized as two polymorphs. The deep-blue crystals of one polymorph are stable upon exposure to air for about a week, while the solution in THF decomposes rapidly when exposed to air. In a side reaction, the different carbene species react with each other under CH activation and CC bond formation in the presence of the biradical.

The linear product is formed as the major product when in situ generated titanium complexes with aminopyridinato ligands are used as catalysts for hydroaminoalkylation reactions of styrenes (see scheme). The reaction is not limited to the use of N-methylanilines and for the first time can be performed with N-alkylanilines bearing alkyl groups larger than methyl, or even with dialkylamines. The best selectivities in favor of a linear product are better than 90:10.

Modern sequencing techniques deliver more and more genetic information about bacterial terpene cyclases. Chemical characterization of these enzymes must stay up to date with these developments. For this purpose a rapid and efficient method for the characterization of bacterial terpene cyclases by heterologous expression in E. coli and direct headspace analysis was developed. The products of six bacterial terpene cyclases have been identified (see structures).

Beyond bulk dielectric relaxation: The experimentally observed time-dependent Stokes shift of a molecular probe (MQ) can be explained by molecular dynamics simulations in combination with DFT calculations. Decomposition of the MD trajectories shows that an important contribution to the time-dependent Stokes shift originates from a group of water molecules that strongly interact with the molecular dipole of MQ.

BLA=0? Not so fast! A series of tetraarylcumulenes up to the length of a [9]cumulene has been synthesized and analyzed by X-ray crystallography. The structural data show a distinct reduction in bond-length alternation (BLA) as a function of molecule length, but this trend appears to reach a limit before a cumulenic structure with BLA=0 is achieved.

The hydrophobic effect is associated with the successive replacement of water molecules in the binding site of a protein by hydrophobic groups of the ligand. Although the hydrophobic effect is assumed to be entropy-driven, large changes in enthalpy and entropy are observed with the model system thermolysin. Structural changes in the binding features of the water molecules ultimately determine the thermodynamics of the hydrophobic effect.

The molecular recognition between cholera toxin B and GM1-functionalized phospholipid/block copolymer hybrid membranes can be controlled by varying the lipid/block copolymer composition. Confocal laser scanning microscopy and fluorescence correlation spectroscopy were used to study the protein–receptor interaction and dynamic processes in the membrane.

Similar yet different: The reaction of a GeBr solution with K[FeCp(CO)₂] (Cp=η-C₅H₅) gives [Ge₁₂{FeCp(CO)₂}₈{FeCp(CO)}₂] (1), whose oblong, highly cross-linked cluster core reveals a cut-out of the high-pressure solid-state structure of germanium (II) (see scheme). Mass spectrometric investigations on the gas-phase species [Ge₆{FeCp(CO)₂}₆{FeCp(CO)}]⁻ (2) and theoretical calculations on 1 and 2 illuminate the formation of the metalloid cluster 1.